In sulphuric acid manufacture from sulphur by the contact process, elemental sulphur is burned in a large excess of dry air in a sulphur furnace to produce a gaseous sulphur dioxide-air mixture at a temperature in the range of 900.degree.-1100.degree. C., containing 8-12% v/v SO.sub.2 and significant, excess oxygen. The downstream conversion of the resultant sulphur dioxide to sulphur trioxide requires 1 mole of oxygen for every 2 moles of sulphur dioxide and since there is a need for excess oxygen to drive the chemical equilibrium to low concentrations of sulphur dioxide there is always much more oxygen than required for stoichiometric combustion of sulphur to sulphur dioxide. The stoichiometric sulphur oxidation would produce a 21% sulphur dioxide gaseous mixture.
Downstream of the furnace in almost all sulphur burning plants is a waste-heat boiler, which cools the sulphur containing gas to, typically, 390.degree.-420.degree. C. for supply to a catalytic converter, while raising steam at pressures in the range 40-60 atm. These steam pressures correspond to boiling water temperatures in the range of 250.degree. to 275.degree. C.
The temperature of the sulphur dioxide containing gas fed to the converter is regulated at present by allowing a portion of the hot sulphur dioxide gas leaving the furnace to bypass the boiler through a hot gas bypass valve. The hot sulphur dioxide containing gas produced in the furnace is, however, very corrosive and few, if any, metallic materials provide a long and reliable life for the valve in the hot sulphur dioxide containing environment. In consequence, furnace temperatures are generally restricted and damper valves at normal design temperatures of about 950.degree. to 1050.degree. C. already have a limited life and require repairs at major plant shutdowns. Typical materials used for the valves include 310, 442 and 446 stainless steel, and several nickel based alloys. Ducting is typically brick lined and a typical valve uses a damper closing on a brick seat having a vertical movement of the valve. Unfortunately, such a valve seat does not provide a tight seal, which results in hot gas bypassing even when the valve is closed. In consequence, the boiler must therefor cool the gas below the desired temperature for entering the first bed of a converter in order to compensate for this hot gas leakage. This results in wasteful additional cooling surface in the boiler to achieve the required cooling.
A further variation on furnace and boiler design systems has air bypassing both the furnace and the boiler, with hot gas also bypassing the boiler. With both air and hot gas bypassing the boiler the temperature of the exit gas leaving the boiler is now higher than in the previous prior art arrangement, for the same converter gas inlet temperature and results in more efficient heat transfer. However, the furnace gas temperature is higher and the corrosive attack on the bypass valve is accelerated as compared to the earlier described system.
Both of the above bypass arrangements represent prior art systems which leave the hot gas bypass valve exposed to very high temperatures where the valve life is limited, despite the use of expensive heat and corrosion resistant alloys.
With plant sizes, steam pressures and boiling temperatures increasing, and lower temperature catalysts available on the market, cooling of the gases between the furnace and the converter by means of the boiler is becoming increasingly difficult as the typical fire-tube boiler used in sulphuric acid plants is becoming difficult to provide in many cases. In addition, the trend to higher efficiency in acid plants also tends to reduce the throughput air used, which further raises the furnace temperature and reduces the life of the bypass valve.